The Container
class provides a convenient way to create a widget with specific properties:
width, height, background color, padding, borders, and more.

Simple animations often involve changing these properties over time.
For example,
you might want to animate the background color from grey to green to
indicate that an item has been selected by the user.

To animate these properties, Flutter provides the
AnimatedContainer
widget. Like the Container widget, AnimatedContainer allows you to define
the width, height, background colors, and more. However, when the
AnimatedContainer is rebuilt with new properties, it automatically
animates between the old and new values. In Flutter, these types of
animations are known as “implicit animations.”

This recipe describes how to use an AnimatedContainer to animate the size,
background color, and border radius when the user taps a button
using the following steps:

Create a StatefulWidget with default properties.

Build an AnimatedContainer using the properties.

Start the animation by rebuilding with new properties.

1. Create a StatefulWidget with default properties

To start, create
StatefulWidget
and State classes.
Use the custom State class to define the properties that change over
time. In this example, that includes the width, height, color, and border
radius. You can also define the default value of each property.

These properties belong to a custom State class so they
can be updated when the user taps a button.

classAnimatedContainerAppextendsStatefulWidget{@override_AnimatedContainerAppStatecreateState()=>_AnimatedContainerAppState();}class_AnimatedContainerAppStateextendsState<AnimatedContainerApp>{// Define the various properties with default values. Update these properties// when the user taps a FloatingActionButton.double_width=50;double_height=50;Color_color=Colors.green;BorderRadiusGeometry_borderRadius=BorderRadius.circular(8);@overrideWidgetbuild(BuildContextcontext){// Fill this out in the next steps.}}

2. Build an AnimatedContainer using the properties

Next, build the AnimatedContainer using the properties defined in the
previous step. Furthermore, provide a duration that defines how long
the animation should run.

AnimatedContainer(// Use the properties stored in the State class. width:_width,height:_height,decoration:BoxDecoration(color:_color,borderRadius:_borderRadius,),// Define how long the animation should take.duration:Duration(seconds:1),// Provide an optional curve to make the animation feel smoother. curve:Curves.fastOutSlowIn,);

3. Start the animation by rebuilding with new properties

Finally, start the animation by rebuilding the AnimatedContainer with
the new properties. How to trigger a rebuild? Use the
setState()
method.

Add a button to the app. When the user taps the button, update
the properties with a new width, height, background color and border radius
inside a call to setState().

A real app typically transitions between fixed values (for example,
from a grey to a green background). For this app,
generate new values each time the user taps the button.

FloatingActionButton(child:Icon(Icons.play_arrow),// When the user taps the buttononPressed:(){// Use setState to rebuild the widget with new values.setState((){// Create a random number generator.finalrandom=Random();// Generate a random width and height._width=random.nextInt(300).toDouble();_height=random.nextInt(300).toDouble();// Generate a random color._color=Color.fromRGBO(random.nextInt(256),random.nextInt(256),random.nextInt(256),1,);// Generate a random border radius._borderRadius=BorderRadius.circular(random.nextInt(100).toDouble());});},);

Complete example

import'dart:math';import'package:flutter/material.dart';voidmain()=>runApp(AnimatedContainerApp());classAnimatedContainerAppextendsStatefulWidget{@override_AnimatedContainerAppStatecreateState()=>_AnimatedContainerAppState();}class_AnimatedContainerAppStateextendsState<AnimatedContainerApp>{// Define the various properties with default values. Update these properties// when the user taps a FloatingActionButton.double_width=50;double_height=50;Color_color=Colors.green;BorderRadiusGeometry_borderRadius=BorderRadius.circular(8);@overrideWidgetbuild(BuildContextcontext){returnMaterialApp(home:Scaffold(appBar:AppBar(title:Text('AnimatedContainer Demo'),),body:Center(child:AnimatedContainer(// Use the properties stored in the State class.width:_width,height:_height,decoration:BoxDecoration(color:_color,borderRadius:_borderRadius,),// Define how long the animation should take.duration:Duration(seconds:1),// Provide an optional curve to make the animation feel smoother.curve:Curves.fastOutSlowIn,),),floatingActionButton:FloatingActionButton(child:Icon(Icons.play_arrow),// When the user taps the buttononPressed:(){// Use setState to rebuild the widget with new values.setState((){// Create a random number generator.finalrandom=Random();// Generate a random width and height._width=random.nextInt(300).toDouble();_height=random.nextInt(300).toDouble();// Generate a random color._color=Color.fromRGBO(random.nextInt(256),random.nextInt(256),random.nextInt(256),1,);// Generate a random border radius._borderRadius=BorderRadius.circular(random.nextInt(100).toDouble());});},),),);}}